Luminescent and Magnetic Properties of Fe₃O₄@SiO₂:phen:Eu³⁺

Abstract

Magnetite was doped with rare earth ions (europium) to produce core-shell materials with both magnetic and luminescent properties, i.e., a magnetic Fe₃O₄ oxide core and a SiO₂:phen:Eu³⁺ shell. The resulting material was analyzed by X-ray powder diffraction and transmission electron microscopy, and subjected to magnetic and luminescence emission measurements. All the synthesized materials exhibited superparamagnetic behavior and luminescence emission. The magnetic behavior of Fe₃O₄ and luminescence emission of SiO₂:phen:Eu³⁺ of the materials were compared to precursors.

Keywords
magnetite; Fe₃O₄; core@shell; luminescence; Eu³⁺

1. Introduction

In recent years, the synthesis of magnetic iron oxide nanoparticles (particularly magnetite - Fe₃O₄) has been intensively explored with a view to biomedical applications such as targeted drug delivery, magnetic resonance imaging (MRI), and magnetic hyperthermia^1-8.

Multifunctional materials are attractive because they may combine properties, which allow manipulation chemical functionalities, playing a key role in different applications. The use of multifunctional materials will, and in some cases already do, allow savings in number of parts, reducing the need for joining operations⁹. Moreover, materials that present both magnetic and luminescent properties have been developed^10-14 and applied to biotechnological processes such as imaging, tracking, and separation of biological molecules or cells^15-19. One way to obtain both properties in a single multifunctional material is by using the core-shell strategy, which involves coating Fe₃O₄ nanoparticles with an actively fluorescent material, resulting in a system with magnetic and luminescent properties^16,20-22. Thus, since lanthanide ions (Ln³⁺) have important applications in fluorescent materials, electroluminescent devices and fluorescent probes for biological systems, they can be used as sources of fluorescence in multifunctional magnetic materials^16,18-22. In fact, they have Laporte forbidden intraconfigurational f-f transitions, and therefore present low absorption intensities. Organic ligands can be used as light collectors (antenna effect). 1,10-phenanthroline or its derivatives have been used as "antennas" of near-UV radiations in Eu³⁺ complexes due to their efficient ligand-to-lanthanide intracomplex energy transfer. The emission enhancement in the sample containing 1,10-phenanthroline is due to that the excitation energy is absorbed by this molecule and then occurs an efficient energy transfer from its triplet state to the Eu³⁺ ions. This excitation process is more efficient that the direct excitation, since the lanthanide cations are characterized by very low absorption coefficients²³.

In core-shell systems, silica is a common capping material, since it is chemically inert, biocompatible, optically transparent and does not affect reactions at the core surface. Furthermore, coating with silica protects magnetite core particles and the luminescent probes can be dispersed on it^6,20,24.

It has been proven that the Fe₃O₄ will greatly decrease the luminescence of Eu³⁺ ion if they are directly blended with them. Therefore, the Eu³⁺ should be effectively isolated to avoid direct contact with Fe₃O₄ if the strong luminescence of the magnetic-fluorescent bifunctional is to be achieved¹³. Thus, Core/shell structure reduces the interaction of the earth-rare ion with the magnetic material. This work involved the synthesis of a magnetic-luminescent core-shell nanocomposite (Fig. 1). The magnetite core was coated with a luminescent silica (SiO₂:Eu³⁺) shell.

Figure 1
Magnetic materials that emit luminescence.

2. Materials and Methods

All the chemicals were of higher than 99.9% purity and were used as received. Magnetite nanoparticles were obtained by the coprecipitation of Fe³⁺(FeCl₃.6H₂O) and Fe²⁺ (FeSO₄.7H₂O) salts (molar ratio of 2:1) in an alkaline medium (pH = 11), using an adapted version of a procedure described elsewhere²⁵. The overall reaction can be written as equation 1:

(1) ${\mathrm{Fe}}^{3+}+{2Fe}^{2+}+{8OH}^{-}\to {\mathrm{Fe}}_3{O}_4+{4H}_{2}O$

To produce the core-shell nanocomposites, appropriate amounts of tetraethylorthosilicate, phenanthroline and europium(III) were dissolved in 20 mL of oxygen-free ethanol. Then, 1g (0.0043 mol) of Fe₃O₄ was added and the system was mixed for 30 min. The precipitate was then washed several times in ethanol and dried at room temperature.

Table 1 describes the composition of the synthesized samples. The composition of the core consists solely of Fe₃O₄ while that of the shell comprises solely SiO₂:phen:Eu³⁺. Sample Fe-Si-1 contains primary proportion of the core and shell compositions; the samples Fe-Si-2 and Fe-Si-3 were obtained by variation in the layer thickness or concentration of Eu³⁺ ions. In all samples the molar ratio of phen and Eu³⁺ was set to be 4:1 as used by Liu and co-authors²⁶ to produce a high luminescent nanocomposite.

Table 1
Summary of prepared samples.

Sample	Fe3O4 (mol)	SiO2 (mol)	phen (mol)	Eu3+ (mol)	Code
Fe3O4	0.0043	-	-	-	Fe3O4
Fe3O4@SiO2:phen:Eu	0.0043	0.0013	0.0008	0.0002	Fe-Si-1
Fe3O4@SiO2:phen:Eu	0.0043	0.0026	0.0016	0.0004	Fe-Si-2
Fe3O4@SiO2:phen:Eu	0.0043	0.0013	0.0016	0.0004	Fe-Si-3
SiO2:phen:Eu	-	0.0013	0.0008	0.0002	SiO2:phen:Eu3+

To determine their structural characteristics, the powders were characterized by X-ray powder diffraction (XRD) in a Shimadzu XRD 6100 diffractometer, using CuKa (k = 1.5406 Å) radiation. The data were collected in fixed-time mode, from 10º to 80º in the 2θ range, using a divergence slit of 0.5º and receiving slit of 0.3 mm and a step size of 0.02º. Microstructural and morphological analyses were performed by field emission scanning electron microscopy (FESEM, Zeiss Supra 35), using 2 to 4 kV under different levels of magnification. The powders were examined by transmission electron microscopy (TEM) in a JEOL 3010 ARP microscope operating at an accelerating voltage of 300 kV. The room temperature nanoparticle magnetization characterization was obtained using a vibrating sample magnetometer Lakeshore model 7300. The PL measurements were taken using a Jarrell-Ash MonoSpec 27 monochromator and a Ge photodetector coupled to a data acquisition system composed of a microcomputer-controlled SR530 lock-in amplifier. The 350.7 nm excitation wavelength of a krypton ion laser (Coherent Innova) was used, with the laser output kept at 200 mW.

All the measurements were taken at room temperature.

3. Results and Discussion

Fig. 2 shows the X-ray diffraction patterns of all the samples.

Figure 2
XRD patterns of samples Fe₃O₄, Fe-Si-1, Fe-Si-2, and Fe-Si-3.

After indexing the peaks of the core (sample Fe₃O₄), the cubic structure of magnetite was identified and found to be in good agreement with JCPDS card no. 089-096²⁷. The crystallite size of the core, determined by XRD from the broadening of the (311) diffraction peak using Scherrer's formula, was found to be 8 nm. The crystal structure of the core did not change after coating with SiO₂. However, the incorporation of silica was identified by the broad peak between 20º and 30º.

Fig. 3 shows a typical TEM image of silica-coated magnetite obtained in sample Fe-Si-1.

Figure 3
TEM image of Fe-Si-1.

In this figure, note the quasi-spherical ~8 nm sized crystalline structures coated with an amorphous phase of heterogeneous thickness. Also, the lattice fringes (2.9 Å) in Fig. 3 agree well with the distance between the (220) lattice planes, also observed in the XRD patterns, confirming the magnetite structure of these crystallite cores. Thus, based on chemical cross-linking, XRD measurements and TEM characterization, it can be concluded that the synthesis method employed here resulted in a polynucleated core-shell system composed of a magnetic core of Fe₃O₄ embedded in a silica shell^28,29.

The magnetization curves of Fe₃O₄, Fe-Si-1, Fe-Si-2, and Fe-Si-3 powder samples measured room temperature are shown in Fig. 4.

Figure 4
VSM data of samples Fe₃O₄, Fe-Si-1, Fe-Si-2, and Fe-Si-3.

Magnetization of the Fe₃O₄ core increased as the applied magnetic field was increased, tending to saturation at high magnetic fields^{16,20,27,28,30}. The saturation magnetization - calculated at the maximum magnetic field - was 48.9 emu/g. This is lower than bulk values but typical for nanosized magnetite, as was found by TEM measurements, and is probably due to cationic redistribution/defects and/or surface effects that affect the magnetization characteristics of nanosized grains. Moreover, this sample displayed features of superparamagnetism, such as negligible remanence and coercivity²⁰, which were observed in the hysteresis loops. The inset of Fig. 4 shows magnetization only for the synthesized samples.

The magnetization of the core-shell samples decreased in proportion to the thickness of the silica coating, which is a diamagnetic material. However, all the coated samples showed the same magnetic behavior as the core. If the magnetization of samples Fe-Si-1, Fe-Si-2, and Fe-Si-3 were normalized by the saturation magnetization of the core, the estimated magnetic mass of these samples would be 18, 11, and 19%, respectively.

The photoluminescence (PL) property of the samples was investigated, as indicated by the spectra shown in Fig. 5.

Figure 5
PL spectra of samples Fe-Si-1, Fe-Si-2, Fe-Si-3, and SiO₂:phen:Eu³⁺ (inset).

All the samples exhibit characteristic ⁵D₀→⁷F_J (J=0-4) Eu³⁺emissions, i.e., they are luminescent materials. The hypersensitive ⁵D₀→⁷F₂ emission is predominant, indicating that the Eu³⁺ ions occupy sites of low symmetry without inversion centers. On weak emissions of the higher ⁵D_J levels are visible, indicating efficient depopulation of the T₁ state through the antenna effect^16,20,31. The photoluminescence emission intensity of the core-shell samples was compared based on the intensity of ⁵D₀→⁷F₂. Samples Fe-Si-1, Fe-Si-2, and Fe-Si-3 were found to present 18, 3, and 3% of PL emission, respectively, relative to the PL emission intensity of SiO₂:phen:Eu³⁺ host.

The low intensity of Fe-Si-2 and Fe-Si-3 samples was attributed to the high concentration of Eu³⁺ ions. When many identical luminescent centers are present a "concentration quenching" may occur. Thus, the excitation energy is lost to the killer sites non-radiatively due to the increase in the number of luminescence centers³². The Fe-Si-2 and Fe-Si-3 samples are twice the amount of europium than sample Fe-Si-1.

4. Conclusions

Core-shell materials with magnetic and luminescent properties were synthesized in this study. The materials are composed of a crystalline magnetic core of Fe₃O₄ and a luminescent shell of amorphous SiO₂. All the samples exhibit magnetization typical of nanosized magnetite and luminescent emission typical of europium(III) ions.

The performance of the Fe-Si-1 core-shell sample is promising. This sample has a crystalline magnetite structure, nanosized, superparamagnetic behavior and luminescent property. Samples Fe-Si-2 and Fe-Si-3 are superparamagnetic and luminescent, but sample Fe-Si-1 shows the best performance.

5. Acknowledgment

The authors thank the Brazilian research financing institutions CAPES and CNPq for their funding of this work.

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Publication Dates

• Publication in this collection
  24 June 2017
• Date of issue
  Sep-Oct 2017

History

• Received
  09 Nov 2016
• Reviewed
  26 May 2017
• Accepted
  03 July 2017